Scientists Still Don’t Know How or When the Grand Canyon Formed. New Research May Hint at Its Ancient Origins

Researchers say the ancestral Colorado River formed an ancient lake in northern Arizona roughly 6.6 million years ago, which spilled out westward onto the landscape that would eventually become the Grand Canyon

Sarah Kuta | Daily Correspondent

April 17, 2026

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The Grand Canyon is one of America’s most popular natural landmarks, attracting more than four million visitors per year. Yet researchers still don’t know exactly how and when the massive gorge formed.

A study published April 16 in the journal Science provides new evidence for one particular theory, the long-debated “lake spillover” hypothesis. The study’s co-authors say their findings support the idea that the ancestral Colorado River formed an ancient lake in northern Arizona roughly 6.6 million years ago. Once the lake filled up, the water then spilled out westward onto the landscape and began traveling along its present-day course, etching out the Grand Canyon in the process.

“It’s a simple but powerful explanation for how the Colorado River system took shape,” says study co-author Ryan Crow, a geologist with the U.S. Geological Survey, in a statement.

Scientists reached this conclusion by collecting 19 samples of sandstone from sites in the Bidahochi Basin, an area just east of the Grand Canyon in northern Arizona. Trapped inside the sandstone samples were thousands of zircon crystals, which the scientists dated using a technique that measures the radioactive decay of uranium into lead.

Zircon crystals, which are incredibly durable, can act as a river’s “fingerprint.” By determining their ages and identifying trace elements within them, scientists can determine when and where they originated.

When the scientists compared the composition and age distribution of zircon crystals from the Bidahochi Basin with those from known, early Colorado River sediments, they found a close match. This discovery suggests that the sandstone in the Bidahochi Basin was once sediment deposited by the Colorado River. The researchers found other evidence to support this theory, including increased sediment accumulation, carbonate strontium isotope ratios and fossilized fish.

Based on the data, they suspect that the proto-Colorado River flowed into the basin and created an ancient body of water called Lake Bidahochi, also known as Hopi Lake, around 6.6 million years ago. Over hundreds of thousands of years, the river gradually filled the lake, until water levels eventually rose high enough to overtop a geologic formation known as the Kaibab Arch. From there, the water continued snaking west.

Other mechanisms, such as groundwater flow or erosion, may have played supporting roles. But the researchers say their findings point to lake spillover as the primary method by which the Colorado River traveled along its current path and carved out the Grand Canyon.

Additionally, although parts of the Grand Canyon may have been “partially carved by other river systems, it was the Colorado River that linked them together,” Crow tells USA Today’s Doyle Rice.

New data aside, some scientists remain skeptical of the co-authors’ interpretation. Rebecca Flowers, a geologist at the University of Colorado Boulder who was not involved with the research, tells Science’s Paul Voosen that the co-authors “make a reasonable case that lake spillover can explain the data.” But she notes that their findings don’t completely rule out other scenarios.

Additionally, Karl Karlstrom, a geologist at the University of New Mexico who was also not involved with the research, tells Scientific American’s Cody Cottier that the “key details” of the study’s proposed lake spillover conclusion “remain untested.”

Exactly how the Grand Canyon formed is still up for debate. However, the new findings may help solve another mystery. Scientists generally agree that the Colorado River existed in western Colorado roughly 11 million years ago and exited the Grand Canyon around 5.6 million years ago. But where did it flow in the interim?

If it formed a lake in the Bidahochi Basin roughly 6.6 million years ago, that might help account for part of the gap.

"Our new evidence shows that it pooled just east of the Grand Canyon, feeding a vibrant ecosystem,” lead author John He, a geologist at the University of California, Los Angeles, tells Live Science’s Stephanie Pappas.

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Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.